In buried heterojunction semiconductor lasers of the multiple quantum well type, each quantum well consists of a comparatively low band gap layer, also called the well, sandwiched between a pair of barriers of higher band gap. When the plurality of quantum wells is very large, it is called a superlattice. Typically, such buried heterojunction lasers were fabricated to be discrete devices, and this allowed the p and n electrodes to be located on opposite sides of the substrate. As a result, the size of the electrodes could be relatively large and was not limited by the width of the active region. In order to facilitate integration of these devices with other electronic circuitry, some prior art heterojunction lasers are designed with both electrodes on the same side of the substrate. However, this configuration implies that the electrode which overlies the laser active region, and hereinafter called the central stripe electrode, be narrower than the active region and accurately positioned thereon.
FIG. 2 shows a cross-section of a prior art heterojunction laser appropriate for integration. In FIG. 2, a p-type AlGaAs cladding layer 202, multiquantum-well (MQW) active layer 203, n-type AlGaAs cladding layer 204 and n-type GaAs contact layer 205 are successively produced on a semi-insulative GaAs substrate 201. Zn is then selectively diffused to create p-type diffused regions 208 such that an n-type region, in a stripe configuration, remains. In addition, in the area where the p-n junction is exposed to the surface, the n-type GaAs contact layer is selectively etched so that p side and n side electrodes 206 and 207 can be produced on the surface of the p-type and n-type regions, respectively.
According to this production method, the MQW active layer 203 is disordered at the Zn diffused areas 208 and becomes an AlGaAs layer of average composition, thereby creating a buried heterojunction laser structure. The operation of this prior art semiconductor laser is described in the following paragraph.
In such a semiconductor laser, two kinds of p-n junctions are provided. The first kind is created at the periphery of active region 209 (where the MQW is not disordered). The second kind of junction is created between the n-type AlGaAs cladding layer 204 located above active region 209 and each diffused region 208. Because the first kind of p-n junction has a diffusion voltage lower than that of the second kind, when a voltage is applied between the p side and n side electrodes, a current flows through the p-n junction located at the periphery of active region 209. As a result, carriers are injected in the active region. Since active region 209 is adjacent on its four sides to AlGaAs having a low refractive index, it becomes a light waveguide, and if the width of active region 209 can be made narrow enough, the laser will oscillate at a stable single mode with a low threshold current. Finally, because this prior art semiconductor laser has both p and n side electrodes located on the same main surface with little step difference, it is therefore in a form appropriate for integration.
In prior art buried heterojunction lasers which are appropriate for integration and which utilize the disordering of a superlattice, the n side electrode is confined to the width of the active region. Referring to FIG. 2, it will be appreciated that if the n side electrode 207 were wider than the central stripe 209, it would overlie part of the diffused region 208 and create an undesirable low resistance conductive path between the electrodes consisting of p-type material in the region 208. Consequently, the width of the active region is directly related to that of the electrode. Because it is preferable to have the width of the active region smaller than 2 .mu.m for single transverse mode oscillation, this implies that the n side electrode must be narrower than 2 .mu.m. Common photolithographic production methods do not, however, readily allow the fabrication of electrodes of a width under 2 .mu.m. As a result, the oscillation mode of semiconductor lasers of the prior art is not very stable and the threshold current cannot be reduced. Even if it were possible to produce, by an advanced technology, an electrode of a width around 1 .mu.m, positioning such a narrow electrode would be quite difficult. Moreover, the electrode resistance would be too large to conduct the current required for continuous laser oscillation.